I know I may be a bit slow here but I recently found out that the 357 sig uses 9mm bullets and not 357 bullets. I am wondering what the reason is. For me half of the fun of the 357 magnum is the bullet verity. Without much looking you can find factory loaded ammo with ammo ranging from 110 grain to 200 grain. I have seen reloads from 90 grains up to 230 grains.

Unfortunately for the 357 sig is limited to 150ish grains at the heavy end.

I am just wondering why they went this route?

Most of this has been answered and answered well I think.

"For me half of the fun of the 357 magnum is the bullet verity."

The .357 Magnum has a much longer case and can handle the extra length of the heavier bullets you mention. Most semi auto rounds are limited in the bullet weights and loads they can handle compared to revolver rounds. The other factor is the bottleneck design of the case. There is less room to properly seat the round than in some straight walled cases. So even had they gone from a 9mm bullet to a true 38 caliber bullet there would still be bullet limitations because of the size of the case and the design.

Another factor, already mentioned, is that wheelguns allow for a wider variety of bullet types than do most semis.

I should point out that a fella likely could resize down some 38 caliber bullets for the .357 Sig if handloading. In handloading for the 38 Super it is possible to use use both 9mm and 38 caliber rounds. Most barrels for the Super these days are .355 rather than the .356 of the original design. But some of that might be undoing a bit what the .357 Sig gets you.